For example, JP-A-57-142003 discloses the following antennas. That is, it discloses a monopole antenna in which a flat-plate type radiation element 1001 having a disc shape is erected vertically to an earth plate or the ground 1002 as shown in FIGS. 16A-1 and 16A-2. This monopole antenna is designed so that a high-frequency power source 1004 and the radiation element 1001 are connected to each other through a power feeder 1003 and the height of the top portion of the radiation element 1001 is set to a quarter wavelength. Furthermore, it also discloses a monopole antenna in which a flat-plate type radiation element 1005 whose upper peripheral edge portion has a shape extending along a predetermined parabola is erected vertically to an earth plate or the ground 1002. Still furthermore, it discloses a dipole antenna in which two radiation elements 1001 of the monopole antenna shown in FIGS. 16A-1 and 16A-2 are symmetrically arranged as shown in FIG. 16C. Still furthermore, it discloses a dipole antenna in which two radiation elements 1005 of the monopole antenna shown in FIG. 16B-1 and 16B-2 are symmetrically arranged as shown in FIG. 16D.
In addition, JP-A-55-4109 discloses the following antennas, for example. That is, a sheet-type elliptical antenna 1006 is erected vertically to a refection surface 1007 so that the major axis thereof is parallel to the reflection surface 1007, and power supply is carried out through a coaxial power feeder 1008, as shown in FIG. 16E. FIG. 16F shows an example where the antenna is configured as a dipole. In the case of the dipole type, the sheet-type elliptical antennas 1006a are arranged on the same plane so that the minor axes thereof are located on the same line, and a slight gap is disposed so that a balanced feeder 1009 is connected to both the antennas.
Besides, a monopole antenna as shown in FIG. 16G is disclosed in “B-77: BROADBAND CHARACTERISTICS OF SEMI-CIRCULAR ANTENNA COMBINED WITH LINEAR ELEMENT”, Taisuke Ihara, Makoto Kijima and Koichi Tsunekawa, pp77 General Convention of The Institute of Electronics, Information and Communication Engineers, 1996 (hereinafter referred to as “non-patent document 1”). As shown in FIG. 16G, a semicircular element 1010 is erected vertically to an earth plate 1011, and the nearest point of the arc of the element 1010 to the earth plate 1011 serves as a feed portion 1012. The non-patent document 1 shows that the frequency fL at which the radius of the circle almost corresponds to a quarter wavelength is the lower limit. Furthermore, it also describes an example where an element 1013 achieved by forming a cut-out portion in the element 1010 shown in FIG. 16G is erected vertically to the earth plate 1011 as shown in FIG. 16H, and that little difference exists in VSWR (Voltage Standing Wave Ratio) characteristic between the monopole antenna shown in FIG. 16G and the monopole antenna shown in FIG. 16H. Furthermore, it also discloses an example where an element 1014, which is formed by connecting an element 1014a, which resonates at fL or less and has a meander monopole structure, to an element with the cut-out portion as shown in FIG. 16H, is erected vertically to the earth plate 1011 as shown in FIG. 16I. Incidentally, the element 1014a is disposed to be accommodated in the cut-out portion. The antenna resonates at a frequency lower than fL because of the element 1014a, however, the VSWR characteristic is bad. In connection with the non-patent document 1, disc type monopole antennas are described in “B-131 IMPROVED INPUT IMPEDANCE OF CIRCULAR DISC MONOPOLE ANTENNA”, Satoshi Honda, Yuken Ito, Hajime Seki and Yoshio Jinbo, 2-131, SPRING NATIONAL CONVENTION of The Institute of Electronics, Information and Communication Engineers, 1992, and “WIDEBAND MONOPOLE ANTENNA OF CIRCULAR DISC”, Satoshi Honda, Yuken Ito, Yoshio Jinbo and Hajime Seiki, Vol. 15, No. 59, pp.25–30, 1991.10.24 in “TECHNICAL REPORTS OF THE INSTITUTE OF TELEVISION”.
The antennas described above pertain to a monopole antenna in which a flat-plate conductor having various shapes is erected vertically to the ground surface, and a symmetric dipole antenna using two flat-plate conductors having the same shape.
Besides, U.S. Pat. No. 6,351,246 discloses a symmetric dipole antenna having a special shape as shown in FIG. 17. That is, a ground element 1103 is provided between conductive balance elements 1101 and 1102, and terminals 1104 and 1105, which are lowest portions of the balance element 1101 and 1102, are connected to the coaxial cables 1106 and 1107. Negative step voltage is supplied to the balance element 1101 via the coaxial cable 1106 and terminal 1104. On the other hand, positive step voltage is supplied to the balance element 1102 via the coaxial cable 1107 and terminal 1105. In this antenna 1100, though the distance between the ground element 1103 and the balance element 1101 or 1102 is gradually increased from the terminal 1104 or 1105 toward the outside, it is necessary to input different signals as described above to the balance elements 1101 and 1102, and in order to obtain desired characteristics, it is necessary to always use three elements, that is, the balance element 1101 and 1102 and the ground element 1103.
In addition, FIG. 18 shows a glass antenna device for an automobile telephone disclosed in JP-A-8-213820. In FIG. 18, a fan-shaped radiation pattern 1033 and a rectangular ground pattern 1034 are formed on a window glass 1032, a feed point A is connected to the core wire 1035a of a coaxial cable 1035, and a ground point B is connected to the outer conductor 1035b of the coaxial cable 1035. In this publication, the shape of the radiation pattern 1033 may be an isosceles triangular shape or a polygonal shape.
Furthermore, US-A-2002-122010A1 discloses an antenna 1020 in which a tapered clearance area 1023 and a driven element 1022 whose feed point 1025 is connected to a transmission line 1024 are provided within a ground element 1021 as shown in FIG. 19. Incidentally, the gap between the ground element 1021 and the driven element 1022 is maximum at the opposite side to the feed point 1025 on the driven element 1022, and the gap therebetween is minimum in the neighborhood of the feed point 1025. The driven element 1022 is equipped with a concavity at the opposite side to the feed point 1025 of the driven element 1022. The concavity itself is opposite to the ground element 1021, and it serves as means for adjusting the gap between the driven element 1022 and the ground element 1021.
As described above, though various antennas have been hitherto known, the conventional vertical mount type monopole antennas have problems that their sizes are large, and it is difficult to control the antenna characteristic since it is difficult to control the distance between the radiation conductor and the ground surface. Furthermore, the conventional symmetrical type dipole antennas also have a problem that it is difficult to control the antenna characteristic since the radiation conductors have the same shape, thereby it is difficult to control the distance between the radiation conductors.
Besides, the special symmetric dipole antenna described in U.S. Pat. No. 6,351,246 has a problem on the implementation, in which a lot of elements and two kinds of signals, which are supplied to the elements, must be prepared. In addition, the ground pattern 1103 is opposite to the balance element 1101 and 1102, but the sides of the ground element 1103, which are opposite to the balance element 1101 and 1102, are straight lines.
Furthermore, JP-A-8-213820 does not disclose and suggest that the outer shape of the ground pattern 1034 is processed.
In addition, though the antenna of US-A-2002-122010A1 aims at miniaturization, the structure that the driven element is provided within the ground element cannot achieve the sufficient miniaturization because of the shape of the ground element. Besides, the shape of the ground element does not have a tapered shape with respect to the driven element.